Chondrocyte maturation is a key step in cartilage development. Defects in chondrocyte maturation lead to significant skeletal malformation, such as osteochondrodysplasias. The long term goal of this proposal is to understand the molecular mechanisms of chondrocyte maturation. For this purpose, we propose to study one naturally occurring murine model, osteochondrodystrophy (ocd). The ocd is an autosomal recessive disorder associated with abnormalities of bone development. Our preliminary data show that by 12 days after birth, the mutant mice exhibit several histologic abnormalities in their growth plates: fewer proliferating chondrocytes, disorganized proliferative chondrocyte columns, and the necrotic cells in the proliferative zone. The mRNA level of the fibroblast growth factor receptor-3 (FGFR-3), a negative regulator of chondrocyte maturation, is shown elevated in the homozygous mutant mice, which suggests that the abnormal chondrocyte maturation may result from consequences of activation of FGFR-3 pathway. We have positioned the ocd gene 0.14 cM centromeric to D19Mit42 on chromosome 19 by linkage analysis and have constructed a physical map of the ocd locus with YAC clones. The aims of this study are to 1) clone and characterize the mouse ocd gene, 2) determine spatiotemporal expression patterns of the ocd gene during embryonic development, and 3) study the biological functions of the ocd gene. For aim 1, the ocd gene will be cloned by cDNA selection and/or exon-trapping and the mutation of the ocd gene will be determined by direct DNA sequence analyses. For aim 2, mRNA expression patterns of the ocd gene will be defined by section- and whole-mount in situ hybridizations, and protein distribution patterns of the gene will be determined by immunohistochemistry. For aim 3, we will first test the hypothesis that FGFR-3 pathway is activated in the mutant mice by examination expression level of p21, a downstream target of the FGFR-3. In addition we will generate new mutant mice carrying ocd and a null-mutation of the FGFR-3 by crossing the ocd mice with the FGFR-3 knock-out mice to delineate the sequence and specificity of action of the ocd gene. Finally, overall function of the ocd gene will be studied by examining a number of genes functionally implicated in chondrocyte maturation, including Indian hedgehog, parathyroid hormone- related peptide, the cell death inhibitor Bcl2, bone morphogenetic protein-6. Result from the proposed research will provide a rational basis for understanding the molecular mechanisms of cartilage growth and maturation.